Thin film photovoltaic (PV) modules (also referred to as “solar panels”) based on cadmium telluride (CdTe) paired with cadmium sulfide (CdS) as the photo-reactive components are gaining wide acceptance and interest in the industry. CdTe is a semiconductor material having characteristics particularly suited for conversion of solar energy to electricity. For example, CdTe has an energy bandgap of about 1.45 eV, which enables it to convert more energy from the solar spectrum as compared to lower bandgap semiconductor materials historically used in solar cell applications (e.g., about 1.1 eV for silicon). Also, CdTe converts radiation energy in lower or diffuse light conditions as compared to the lower bandgap materials and, thus, has a longer effective conversion time over the course of a day or in cloudy conditions as compared to other conventional materials.
The junction of the n-type layer and the p-type layer is generally responsible for the generation of electric potential and electric current when the CdTe PV module is exposed to light energy, such as sunlight. Specifically, the cadmium telluride (CdTe) layer and the cadmium sulfide (CdS) form a p-n heterojunction, where the CdTe layer acts as a p-type layer (i.e., an electron accepting layer) and the CdS layer acts as an n-type layer (i.e., an electron donating layer). Free carrier pairs are created by light energy and then separated by the p-n heterojunction to produce an electrical current.
Higher performance has been seen in such modules when using a cadmium sulfide layer sputtered in the presence of oxygen. This oxygen in the deposition process correlates to oxygen being included in the cadmium sulfide thin film layer, in the form of a random mixture of compounds that include CdS, CdO, CdSO3, and CdSO4. However, the exact stoichiometry of the deposited layer cannot be controlled in this reactive sputtering process.
Thus, a need exists for controlling the oxygen content and stoichiometry of the cadmium, sulfur, and oxygen forming the n-type window layer for use in a cadmium telluride based thin film PV device.